Berlin, Germany | A loyal ally and fierce critics are among the contenders to succeed Germany's Angela Merkel as head of the centre-right CDU party and secure a future shot at becoming chancellor.

Here's a look at the top candidates.

- Crown princess: 'AKK' -

Known as "mini-Merkel", 56-year-old Annegret Kramp-Karrenbauer -- the former premier of tiny Saarland state -- is a loyal follower of the chancellor's centrist line that has shaped the CDU and German politics over the past two decades.

At Merkel's initiative, she became party general secretary in February, the number two spot offering a leg-up towards taking the helm.

Often referred to by her initials "AKK", her Catholic views overlap more with the conservative wing of the CDU than Merkel's on social questions such as abortion or gay marriage.

Throughout the campaign she has played up her long experience in office and her history of winning regional elections, a marked contrast with her two male competitors.

Polls show Kramp-Karrenbauer is the most popular candidate for average voters -- but it is unknown what weight this will carry among the 1,001 CDU delegates who will vote in Hamburg on Friday.

- 'Anti-Merkel': Jens Spahn -

Right-winger Jens Spahn, a hardliner on immigration, is seen by many as the "anti-Merkel".

The chancellor named her critic Spahn as health minister in her fourth government to appease the CDU's conservatives, but he hasn't held back from attacking what he sees as her overly "Social Democratic" party line.

He has sharpened his profile through meetings with right-wing darlings like Austrian Chancellor Sebastian Kurz and US ambassador to Berlin Richard Grenell, a prominent supporter of President Donald Trump.

A picture of Grenell and Spahn, who are both openly gay, posing with their respective spouses after a casual dinner in Berlin made headlines in Germany.

But many see the leadership contest as having come too early for the ambitious 38-year-old, who has seen supporters desert him in favour of Friedrich Merz despite his attempts to strike a tough tone on migration.

- Avenger: Friedrich Merz -

Corporate lawyer Friedrich Merz, 63, has never forgiven the chancellor for driving him out as head of the party's group of MPs in the Bundestag in 2002.

Like Spahn, financial policy expert and social conservative Merz has complained that Merkel led the CDU too far to the left.

He quit the Bundestag following his defeat at her hands, returning to work as a lawyer and heading the supervisory board of mammoth asset manager BlackRock's German arm.

While he is the clear favourite of pro-business forces in the party and has drawn conservatives away from Spahn with a promise to "halve" the vote share of far-right Alternative for Germany (AfD), gaffes have marred his campaign.

Early on, he described himself as a member of the "upper middle class" -- only to admit soon after that he earns around one million euros ($1.1 million) per year.

And he was widely criticised for a suggestion to weaken the constitutional right to asylum in the wake of the 2015-16 refugee influx that has shaken German politics under Merkel.